1. Field of the Invention
This invention relates generally to protective relays, and more specifically to protective relays employing a directional comparison blocking scheme.
2. Description of the Prior Art
Electrical transmission lines and power generating equipment must be protected against insulation faults and consequent short circuits that could cause collapse of the power system, serious and expensive apparatus damage, and personal injury. For instance, such a fault is caused by lightning-induced flashover from a transmission line to ground or between adjacent transmission line conductors. Under such a fault condition, line currents can increase to several times the normal value, thereby causing loss of synchronism among generators and damaging or destroying both the transmission line and the attached equipment. To avoid equipment damage and collapse of the entire power system, faulted apparatus on the main transmission line must be isolated from the network in the range of approximately 0.1 to 0.5 seconds. The isolation time limit must allow for the operation of large circuit breakers, interrupting up to 80,000 A, and completion of back-up operations if these primary protective devices fail to function properly. To allow sufficient time for circuit interruption, location of the fault must be determined in approximately 8 ms to 20 ms. It is the function of the protective relays, which monitor ac voltages and currents, to locate line faults and initiate isolation via tripping of the appropriate circuit breakers. These faults are located by determining the relationship of the ac voltages and currents.
One type of protective relay system for transmission lines employs a protective relay apparatus at each transmission line terminal. Analyzing the ac voltages and currents, each protective relay operates independently to determine if a fault has occurred on the intervening protected line segment. This type of protective relay system usually provides high-speed simultaneous protection for only 70% to 80% of the faults in the middle section of the protected line segment. For faults occurring near the ends of the line, the protective relay nearest the fault clears the line at high speed. The relay remote from the fault, however, clears the line with a time delay unless system conditions provide high-speed sequential tripping.
In another type of protective relay system, known as pilot relaying, the protective relays at each end of the transmission line communicate with each other via a bidirectional communications link. The relays at each line terminal compare fault location information to quickly determine if the fault is on the intervening line segment. If the comparison indicates that the detected fault is internal, i.e., between the two protective relays, the intervening transmission-line section is isolated by tripping the circuit breakers at the line terminals. If the comparison indicates the fault is not between the two protective relays, the circuit breakers remain closed and the line operative. Pilot protection provides high speed protection for 100% of the protected line independent of system conditions. The communications channel linking the two relays can employ a modulated carrier signal transmitted over a power line phase conductor or over a microwave system, or audio tones propagated on a leased telephone line or on a dedicated pilot wire.
There are several advantages to high-speed simultaneous clearing at all terminals for all line faults, including: reduced possibility of line damage and conductor burndown; improved transient stability of the power system, and provisions for the use of high-speed reclosing, which if successful, improves transient stability, minimizes outage time, and improves voltage conditions on portions of the system load.
There are many different types of pilot relaying systems; the present invention includes features of a blocking pilot relaying system. In the traditional blocking system, the communications channel linking the protective relays at opposite ends of the protected line segment is used only to prevent one or more of those protective relays from tripping on an external fault. A channel signal is not required for internal faults, i.e., tripping occurs in the absence of a channel signal. When a channel signal is present, the protective relays are blocked from tripping.
To detect a fault with a pilot relaying system, either directional comparison relays or phase comparison relays are used at each terminal. In the directional comparison scheme, fault detecting relays compare the direction of power flow at the two line terminals. Power flow into the line at both terminals indicates an internal fault and the circuit breakers at each terminal of the line are tripped. If the power flows into the line at one terminal and out of the other, the fault is external and the line remains in service.
More specifically, the prior art directional comparison blocking scheme employs four relays at each line terminal to detect phase and ground faults and their direction with respect to the relays' location. One of these relays is a phase forward-reaching tripping relay and another is a ground forward-reaching tripping relay (designated 21P and 21NP, respectively). One or both of these relays is responsive to all faults on the protected line segment between the line terminals. The other two relays, for monitoring that portion of the line behind the terminal, are a reverse-reaching phase relay and a nondirectional ground overcurrent relay (designated 21S and Ios, respectively).
In a directional comparison blocking system, if fault current flows into both transmission line terminals simultaneously, no blocking carrier is transmitted and high speed tripping of the breakers occurs at both terminals of the faulted line. For external faults, at one terminal neither the phase nor the ground directional distance relays detect the fault. This prevent tripping at that terminal and transmits a blocking carrier signal to the other terminal. Note that the relays at the latter terminal detect the fault, but upon receipt of the blocking carrier signal these relays are blocked from tripping. Transmission of the blocking carrier signal is initiated by the reverse-reaching phase relay and/or the nondirectional ground overcurrent relay, which are set to reach beyond the tripping relays located at the other terminal.
The feature that distinguishes the directional comparison blocking system from other pilot relaying systems is that the carrier is normally off, and that a sustained blocking signal is transmitted upon the occurrence of an external fault. The fundamental concept of this prior art system is that tripping occurs if either of the directional tripping relays operate and a carrier signal is not received. The prior art directional-comparison blocking scheme is widely used due to its flexibility and reliability. Since the communications channel is not required for tripping, internal faults that might short and interrupt the channel are not a problem. Overtripping occurs, however, if the channel fails or blocking is not established for external faults within the reach of the tripping relays. Since the carrier transmitter is normally off, channel failure cannot be detected until the system is tested or an external fault occurs.
The phase comparison blocking scheme generally uses overcurrent fault detecting relays to compare, via the channel, the relative phase of the currents at the terminals. In one such prior art arrangement, if the currents at the terminals are relatively in-phase an internal fault is indicated, blocking carrier is not transmitted during the negative half-cycle of the ac signal, and the circuit breakers at both terminals of the line are tripped. If the currents at the terminals are relatively 180.degree. out-of-phase, an external fault (or through load current) is indicated and the breakers are not tripped.
Another prior art directional comparison pilot scheme developed to overcome some of the disadvantages of the directional comparison blocking system is a directional comparison unblocking system. The directional comparison unblocking system transmits a continuous blocking signal except during an internal fault. Only two relays are required at each line terminal, a forward reaching phase relay and a forward-reaching ground relay. The communications signal between the line terminals is usually a phase shifted power-line carrier. A blocking (guard) signal is transmitted continuously during normal conditions. For an internal fault the phase-shifted carrier transmitter is shifted to the unblocking or trip frequency, thus tripping circuit breakers at both terminals of the protected line segment.
Like the directional comparison unblocking system, the prior art overreaching transfer-trip system also uses two protective relays at each line terminal; both are forward-reaching distance relays set to overreach the other line terminal. Usually a phase shifted audio tone modulated on a communications channel (microwave or telephone lines) is used with the overreaching transfer-trip system. The relay seeing an external fault transmits a trip signal to the other terminal, but since the latter does not see the fault no trip occurs. For an internal fault the relays at both terminals see the fault, and transmit the corresponding trip signal to the other terminal, thereby tripping the protected line segment. Because transmission of a signal is required to trip, dependability of the overreaching transfer-trip systems is less than that of a prior art blocking system.